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  JS 性能优化
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  <time datetime="2021-04-27T10:20:39.000Z" itemprop="datePublished">2021-04-27</time>
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  <p>javascript 中内存的分配以及回收都是自动的，并未向开发者提供相应操作的 API。但是了解内存的回收机制，对于我们写程序时优化代码有很大的帮助。</p>
<p>内容摘要：</p>
<ul>
<li>垃圾回收 GC 算法</li>
<li>V8 引擎的垃圾回收机制</li>
<li>监控内存性能的工具</li>
<li>性能在页面上的表现</li>
<li>代码的优化</li>
</ul>
<span id="more"></span>
<h2 id="一-GC-算法"><a href="#一-GC-算法" class="headerlink" title="一. GC 算法"></a>一. <code>GC 算法</code></h2><p>垃圾回收算法主要有 4 中：</p>
<h3 id="1-引用计数算法"><a href="#1-引用计数算法" class="headerlink" title="1. 引用计数算法"></a>1. 引用计数算法</h3><ol>
<li><p>核心思想</p>
<p> 计数器</p>
</li>
<li><p>原理</p>
<p> 当引用发生改变时，去改变计数的值，当值为 0 时，会被立即回收。</p>
</li>
<li><p>优点</p>
<ol>
<li>当计数器为 0， 可立即被回收</li>
</ol>
</li>
<li><p>缺点</p>
<ol>
<li>耗时。因为存在计数的逻辑，每次引用发生变化，都需要去更新计数</li>
<li>局部变量中的循环引用，在执行后无法被回收</li>
</ol>
</li>
</ol>
<h3 id="2-标记清除算法"><a href="#2-标记清除算法" class="headerlink" title="2. 标记清除算法"></a>2. 标记清除算法</h3><ol>
<li><p>核心思想</p>
<p> 分标记和清除两阶段</p>
</li>
<li><p>原理</p>
<p> 标记阶段：遍历所有的对象，找到可达对象（从根节点出发可找到的对象）并标记<br> 清除阶段：遍历所有的对象，清除未标记的对象</p>
</li>
<li><p>优点</p>
<p> 可清除局部作用域中的循环引用</p>
</li>
<li><p>缺点</p>
<p> 空间碎片化</p>
<h3 id="3-标记整理算法"><a href="#3-标记整理算法" class="headerlink" title="3. 标记整理算法"></a>3. 标记整理算法</h3><p>标记整理算法的标记阶段和标记清除相同。不同点在清除之前会对标记对象进行整理，使其内存空间连续。这样可以使清除后的空闲链表更好的充分使用。</p>
<h3 id="4-分代回收"><a href="#4-分代回收" class="headerlink" title="4. 分代回收"></a>4. 分代回收</h3><p><code>V8 引擎</code> 使用的就是分代回收思想。下面会详细介绍。</p>
</li>
</ol>
<h2 id="二-V8-引擎"><a href="#二-V8-引擎" class="headerlink" title="二. V8 引擎"></a>二. <code>V8 引擎</code></h2><p><code>V8 引擎</code>是 javascript 的执行引擎，其内存大小有限制（64 位的操作系统内存为 1.5G，32位的操作系统内存为 800M）。V8 引擎的垃圾回收采用分代回收的思想，即分为新生代（64 位的操作系统内存为 32M，32位的操作系统内存为 16M）和老生代（64 位的操作系统内存为 1.4G，32位的操作系统内存为 700M）。由于内存的限制，所以针对不同的代采用不同的高效垃圾回收算法。</p>
<h3 id="1-新生代"><a href="#1-新生代" class="headerlink" title="1. 新生代"></a>1. 新生代</h3><p>新生代的内存空间分为两个相等大小的空间：From 和 To。</p>
<ol>
<li><p>采用的回收算法</p>
<p> <code>标记整理 + 空间赋值</code></p>
</li>
<li><p>新生代晋升为老生代</p>
<ol>
<li>To 空间的使用率超过 25%</li>
</ol>
</li>
</ol>
<h3 id="2-老生代"><a href="#2-老生代" class="headerlink" title="2. 老生代"></a>2. 老生代</h3><ol>
<li><p>采用的回收算法</p>
<p> <code>标记清除+标记整理+增量标记</code></p>
<p> 当执行内存清除时，程序是停止执行的。为了优化用户体验，使用增量标记使得集中的内存清除工作分为多个片段，与程序穿插执行。</p>
</li>
</ol>
<h2 id="三-监控内存性能的工具"><a href="#三-监控内存性能的工具" class="headerlink" title="三. 监控内存性能的工具"></a>三. 监控内存性能的工具</h2><ol>
<li>任务的资源管理器可以监控 DOM 内存和 js 栈内存</li>
<li>Timeline 时序图</li>
<li>堆快照查看分离 DOM</li>
<li>频繁的内存回收可通过 TimeLine 内存走势分析</li>
</ol>
<h2 id="四-性能在页面上的表现"><a href="#四-性能在页面上的表现" class="headerlink" title="四. 性能在页面上的表现"></a>四. 性能在页面上的表现</h2><ol>
<li>随着时间的延长页面性能越来越差（内存泄露，内存越来越满）</li>
<li>页面延迟或者页面经常性的卡顿（内存回收频繁，内存回收时，程序停止执行）</li>
<li>不同的设备上程序性能都很差（内存膨胀，内存申请的空间大于太大）</li>
</ol>
<h2 id="五-代码的优化"><a href="#五-代码的优化" class="headerlink" title="五. 代码的优化"></a>五. 代码的优化</h2><ol>
<li>使用变量暂存全局对象</li>
<li>使用原型添加方法优于使用 class 中的方法</li>
<li>数组的循环建议使用 <code>forEach</code></li>
<li>字面量创建对象优于 <code>new Object</code></li>
<li>for 循环中提前获取长度优于每次遍历去获取长度</li>
<li>减少判断层级（ 通过 <code>return</code> 减少层级嵌套判断。当值为某个确定的值时，可使用 <code>switch...case</code> 代替 <code>if...else if...else</code>，增强可读性 ）</li>
<li>减少作用域链的查找层级</li>
<li>减少对象类型的数据读取次数 （ 对象的读取相对于原始类型会慢 ）</li>
<li>减少循环体中活动</li>
<li>减少声明以及语句数</li>
<li>事件委托绑定</li>
</ol>
 
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